Pale Moon is a customised version of Firefox, which has been carefully optimised for speed and efficiency.
The program has been made more lightweight, for instance, by stripping out little-used components like the accessibility features, and the parental controls. The crash report has also been stripped out, as it's designed to work with server-side technology that isn't available on palemoon.org. And Pale Moon also drops support for Internet Explorer's ActiveX and ActiveX scripting technology, which also offers a security benefit as it means the browser can't be infected by malicious ActiveX controls.
Other optimisations are more technical. In particular, Firefox is compiled with the most conservative of settings, to ensure that it'll run on even ancient CPUs. Pale Moon, though, is optimised to take full advantage of modern processors, and this can give it a huge advantage over Firefox in some areas.
So how much faster can the browser be? That's a tricky question, as performance varies greatly depending on what's being tested.
The browser does have one potential down side, in that it may be incompatible with some extensions. If they've assumed that the browser's program name is firefox.exe, say, or they're using components that Pale Moon has stripped out, like the Parental Controls, then you'll probably find they don't work.
This is rare, though - more extensions install and run just fine. Trying out Pale Moon also gets you access to useful extras, like the Language Packs (you can now run the program in more than 70 languages), a portable edition, and a 64-bit version. And as you can install and run both Pale Moon and Firefox together on the same system, the program is really easy to evaluate. So if any of this sounds right for you, then give Pale Moon a try, and see how much faster it might be on your system.
Pale Moon 27 is a major upgrade. For a detailed list of changes, refer to the release notes.